Coated abrasives



Nav' 2451.942515. N. OGLESBY E? AL 2,302,711

l l l COATED ABRASIvE 1 ffF-led Feb'.' 6, 1937' Mig; if. ogemj 27mm/mSMI@ Patented ,Nov. '11942 U'Nirao L D i i @PATENT zasnzgmfcoazranfannasivas Nicholas "Gglesby `anni'F.Franldin Strain, Troy,-N..,z:assizncrs=.to'BehrManningCorporation,

"sTroy,'1N. `,Y.,::a1:corpuration :of :Massachusettsnppiicai'mmr'ebmary, .11937, serial No.'*124,'5os

@51:3 Claims. (1l-7l. '5l- 298) This invention yrelates Vto`:theY,.nianuiat'sture lo! coated abrasives. and ywith #regard vsito'zits 'more specic features to correcting thericurling tot y a coatedabrasive *which`- willy the f'sumciently .Silat l* for practical usev.at :customarytemperatures 'and results in a change in the .quantity.of .moisture held within .the cellulosic backingvand :the glutiynous or.other hygroscopic adhesivexi'fused. :In the rcase of. a yglutinousbinder,.the dimensions of z both the backing and thelabrasiveeadhesiveccatg :ing change appreciably withchanges `in the .at.

-mospheric moisture, that is, KAwith the Amoisture lcontentsoi ythe.backing and of 'ther'.glutinous or' I Aother hygroscopic adhesive.

In the vcase .of coated abrasivesmade 'witha synthetic resinous binder,suchas the phenol#4 -aldehyde resins. the vvalkydy resins :and :the.urea .resins, there isilittle, if any, change in thedimensions ofthe.abrasive-binder'coatingwithv changes humidities encounteredin'zuse.Stillanot'her'object of this invention istheiprovision l'rit-svc.;process for shrinking the `rreinforcing iback-ing-toffdime'nless thanthose o't lthe abrasiveiafdhesive coating. A further objectoifthepresentfinventionais .torelieve the strains fsetuprbetween#ther-reinforcing backing andthe abrasive`adhesivecoatingwhich ordinarily causing curling, warping'faridffothery such undesirablephenomena. *Asspeciicaobject of the present inventionis'thefcontrolsotzthezcurl of coated abrasives '.rna'defvvith"non-."hygroscopic binders such as synthetic-resins; e. Lg.,fthephenolic resins, the alkyd resins .andureafcrmaldehyde resins.

Another object'of the `inventionris .toipro'v'ide y a product with oneor more-of 'the'vzadvantages cited. Another objectsoftheninventionis"torprovide' a process bywhich-one-fforimore'"ofi-,the.fad-

vantages indicated 1 may abe `:n.ttainef'd.

This invention accordinglyconsistsfinthesfeatures of construction,combinationsfoffieiements, and arrangements of parta-'fanduinftheseveralsteps and relationand orderl'oiieachf'o!'said'steps yto one or morelofthe othersvthereohuallfaswill be illustratively describedherein,.zand'fthezgscope of the applicationfof whichAfwillbe..indicatedsiii the following claims.

In the patent 'tof Nicholas 'glesby, fNo.

2,136,150 issued 'Novemberf8, 1938, the general theory and explanationof'lthe curling-.f'of'rcoated iin atmospheric moisture. lonce the:resinousnfad- "l'hesive has been. lti'ioroughlv cured and .renderedinsolubleinrand resistant tomoisture. vThe-paper backing for otherfcellulosic backing on lfthe .other hand attempts to exert its .usualdimensional sions which are approximatelyiequalfto.zorrslightly 20changes with gain or lossof moisture. In the ypatent kto Nicholas E.'Oglesby, lNo.

2,136,150 issuedV November I8. 1938, `Va vmethod `of decreasingyand-controlling the curl ofrsynthetic resin bonded coated abrasiveshas'ibeenf.discussed f and consists of using paper 'with .ahighcross-tolengthfvstrengtl'ly ratio which in'turn hasta l*low dif-Aferential .of expansionin the rmachine and lcross Y, directions ascompared ywith :normal :paper-.used .inzthe past, which has ordinarilyhada high'dif- 'l ferential of expansion betweenthelengthformachine, andlthe cross-directionof 'the web'.

While the use of paperrandithe procedure de- '-f-scribed inthese-'pending applicationihasbeen a abrasives has been'siven.v@As*applied"to-both 'i coated abrasives made --with:a's'alutinousfbin'der and those madefwith a syntheticfresinf'binder,

the phenomenon of curlingvffcan beifattrlbutedf to unequal expansionsfof 'the' -gadhesive-abrasive coating on the `one :hand .and the`papeivonother reinforcing vbacking` on wwe-:other chand, `withchangesin atmosphericfmoisture,fhwhichiiin-mm ydistinct advance overthetprior art, fthis :method -still does'not solve all theproblexns ofcurling, i. vespecially the'problem lof usingspaper'withahig'h"diil'erential of expansion betweentthe length'fand :cross ydirectionsand at ithesame `time securing ythe'. desired degree of.controlotzcurling'with rnonhygroscopiczbinders.

Even though paper with .a cross and length 'expansion yis .usedQthe`product still curls l`to some-extentandwhat isrmore serious.' the curlis withthe abrasive on thexconcave surface; such an abrasivefmay be:designated as having sand concave-curl. -.'Ihisftype .otfcurl forpractical use is 4generallyzmore vtroublesome :than a sand conveiucurlin which-'the abrasiveis von thezconvex surface of `the coated paper.Furthermore, by means Aof the .present'invention-.itiv

becomes possible kto use `.paper which zwould :not

otherwise be .satisfactory-.because of its excessive vcui-line:characteristics-.in the, absenceorour Yspecial process.

11nl vcarryingvv out f the inventionpwef'make :use `of iow differentialcf sive grains and the two are passed simultaneously'v and in closeproximity between electrodes constituting a field, preferably anelectrostatic field.v The effect of the field is to raise the grainsagainst gravity and space them approximately equi-distant from eachother with their long axes substantially perpendicular to the backing.

This orientation for many purposes gives the mosteffective performanceof the nal abrasive product. -It is a feature of the presentinvention tocontrol the curl of the abrasive so that the oriented abrasive grainsmay be more effectively presented to the work.

If, by way of illustration, fibre backed discs are manufactured with asynthetic resin binder as described in the patent to Nicholas E.Oglesby, No. 2,184,896 issued December 26, 1939, or if alurniuous oxidepaper is manufactured with a synthetic resinous binder as described inthe said copending application, it will be found that when the finalcoated and cured product is exposed to certain atmospheric himidities, asand concavecurl will occur which is objectionable in both cases. Thetemperatures required for baking the resins used by way of illustration,bring the paper to a relatively dry condition and associated with thisdrying of the paper is a corresponding shrinkage in the paper while theresinous bond is still plastic and free to adjust itself more or less tothe dimensions of the contracted papers. After the resin is hardened andisno longer plastic, the paper or other backing is still in a relativelydry condition. When exposed tothe atmosphere. the backing absorbs waterand expands while the abrasive-adhesive coating is little affected byatmospheric moisture. Furthermore, when the coated abrasive made withthese resins is removed from the high temperatures required to cure theresin there is a thermal contraction of the resinous-abrasive coatingwhich is generally in excess of the thermal contraction of thecellulosic backing. The results oi' either or of both of the foregoingphenomena is a sand concave curl and this curl will be found to takeplace to some extent regardless of the characteristics of the paper orsimilar cellulosic backing being used.

When a coated abrasive after fabrication is exposed to variablehumiditles such as occur with changes in the weather and with changes asbe tween indoors and outdoors, there is a change in the curl, sincethere is a change in the dimensions of the cellulosic backing which isnot identical with changes in the dimensions of the acl--hesive-abrasive coating. In the case of coated abrasives made with aresinous binder there is substantially no expansion and contraction ofthe resinous-abrasive coating. The variation in curl resultingfinsuch aproduct withfvariation in humidity is thecurl brought about byv theexpanslon or contraction of the cellulosic backing. The variation incurl after fabrication isminimized by the use of paper with a lowdifferential of cross and length expansion; i. e., high cross-t0- lengthstrength ratio as explained in the patent to Nicholas E. Oglesby, No.2,136,150 issued November 8, 1938.

In the case of coated abrasives made with a glutinous or otherhygroscopic binder, both the cellulosic backing and theadhesive-abrasive coating expand and contract with changes in at-`mospheric moisture, ,that is, with changes in the 'moisture contents of'the backing and coating respectively. In the case of close coatabrasives made with a glutinous adhesive, the troublesome curlingencountered is normally sand convex while in the case of open coatabrasives made with the same type of a. binder the troublesome curlingencountered is usually sand concave, since in this latter case theadhesive-abrasive coating expands or contracts at amore rapid rate thanthe cellulosic backing.

The problem of controlling curl is therefore the` problem of --producinga coated abrasive which is either flat at some average moisture contentthat will be encountered in use or which will show a desirable ratherthan an undesirable curl at the average moisture content encountered inuse. For instance, it is sometimes desirable that the coated abrasivehave a slight degree of sand convex-curl as for instance inthe case ofdiscs, as a slightvr sand convex-curl will yhelp to hold the discrigidly against its support when it is mounted by a mechanical deviceoperating through a center hole in the disc as is customary in manycommercial disc sanding machines. By the term sand convex-curl, asapplied to a disc. it is meant that a disc formed from the abrasive isconvexly curved on its abrasive surface so that a particular diameter isa substantially straight line, while a diameter intersecting thefirst-mentioned diameter at an angle of is substantially an arc of acircle.. Sand concave curling is ordinarily objectionable, and has noneof the advantages of either a flat or a sand convex coated abrasive.

A feature of the present invention is the pro.- vision of a means ofshrinking or otherwise controlling or adjusting the final dimensions ofthe backing to such an extent that under ordinary conditions of use i.e., common relative humidities and temperatures, the said dimensions areequal to or slightly less than the dimensions oi' the abrasive adhesivecoating under the same conditions, whereby the coated abrasive willeither be relatively flat or show a controlled and desired amount ofsand convex-curl. This control or adjustment is accomplished bysubjecting the cellulosic backing to treatment with a volatile swellingagent for cellulose as will be hereinafter described.

As explained before', a coated abrasive made withv a cellulosic backingand a non-hygroscopic binder varies in curl when exposed to variableconditions of humidity and temperature.

In the patent Nicholas E. Oglesby, No. 2,136,150, issued November 8,1938, a method of using a backing with low differential between crossand length expansion with variations in humidity has been disclosed. Aproduct made with this character of cellulosic backing will be subjectto less variation in curl with variations in atmospheric conditions thana backing made with an ordinary cellulosic backing which has a higherdifferential between cross and length expension with changes inatmospheric conditions. 1

In one aspect, the present invention contemplates the use of a backingwith a low differential between cross and length expansions, tominiasoman mize changes in curl with changes ini atmos phericcondition.r With extreme variations in moisture, there arecorrespondingextreme'variations in curl, but these extremevariationsf'willbe a function of the cellulosic. backing character.'-

lstics.

A practical solution of thel problem of control ling the curl of coatedabrasives madev with? a, non-hygroscopic binder and lacellulosicnbaclrlng.v necessitates theproductlon of a coated-abrasivearticle which is free from. a material degreev'of sand concave-curl andiseither relatively ilat. or shows a moderatev and controlled degreerofvsand convex-curl under common variations in. f

15.". For most purposes.` this result". is'. at,-

atmospheric conditions that are encounteredin practice. tained' when thecoated abrasive isfeith'erssulzrstantially flat or shows av moderatedegree of 'n sand convex-curl betweenrelative'humidities ofv 15 and 60%and at work-ingsroom.A temperatures such as, for example, '10v FL Suchlimitationsoi'. humidity include commonl or average-.conditionsin use,i. e., common or averagevariationszof"thegv atmospheric4v conditions orrelati'vef` humidity.' Where reference is made in -thespecification-'and claims to commonor average conditionsqof'use, or tocommon or average kvariations insrelative.v humidity, is meant that"theprod'uct exhibitsrtheff characteristics specifiedvlargelyfthroughout' the humidity range abovesetf'fort'h. In :otherWords;`

does not have a concave abrasivesurfaceunder ordinary workingconditions:

We have provided a meansof shrinking' the cellulosic backing suchl asv awaterlaldfsheet of paper, vulcanized fibre" andl the-like or a; wovenfabric such as cotton clothaorfae combination of a woven and water-laidbacking.v The` exact means by which our process is carriedrout'varies:somewhat according to.- the riaturelof^` thematerials being used andthe.v manufacturing proaess. i.

By "cellulosic" or paper'backing..Imean'= ordinary paper suitable forcoating abrasives` asv fl'i well as suitably treated papers.v forexampl'c.f.vu1l` canized fibre, also laminatedv backings containing-vone of the aforesaid paper materials combined with eithercloth oranother`- paper. or Vcloth'.

Referring to the drawing:

Figure 1 is a sectionalviewof acoatedabr-asive having a sandconcavecurl.

Figure 2 is a sectional view of a. coated abra sive which is devoid. of'curl.v

Figure 3 is a sectional view of a. coatedi abrasive having'a sandrconvex-curl, in; whiclijh', the altitude, is used as a measure .ofy thedegree `oi'.y convex curl. l

'Figure 4 is a diagrammatic.:View'showingtonefv method of carrying. outthe present invention.. and

employed in connection. witlfrFigurev 4a.

Coated abrasive Example'-y L In this case we mayuse by way." ofillustration",

a binder which caribe-cured; at'` a' temperaturebelow the boiling pointof` water: An--example-of such a binder may be preparedbycookin'fcan'oil'.

soluble phenolicY resin'.` suchasBalrelite` resin.

No. 254 with China-wood. oilasis well",v known: in.` the varnish makingart" to" produce`A a. binder which -ior some uses may contain-'f aboutv40% Bakelite resin and: 60% Chin'afwoodoil.` Suit-'-` able driers suchas a manganese. drier,y as: wel-l'I known in the art', may be added;

3013v the abrasive product is' ofy a`y character-.which 6 Figure 5 is adetail. view of. the spray. device" We mayalso use a binder consistingof -Bakelite resin #254, 35%, analkyd resin 30% (suchas. "RezyP 408,obtainable from the American CyanamidCompany) and China-wood oil 35%.

;/Rezyl 408'consists of about 50% oil acids and cosityof body 0 whenthinned to 60% with high flash naphthaA andv tested at a. temperaturevof 77 F.

By way oi' illustration, we'mayr useeither of these, or similarbinders'on a conventionalsandpaper machine as illustrated in thepatentto Nicholas E.4 Oglesby, No. 2,184,896, issuedl December 26,1939to coat No. 320C silicon carbide paper.

whichis prepared in accordance with the needs of" the present sandpapervart, and a. suitabletreatment of the paper on the coat side beforecoating* is heavy4 blown China-wood oilwhich is addedtozlthe extent 'ofabout 15% of thepaper weight to' seal the poresoi.'v thepaper andprevent penetration of the binder'applied to hold" the abrasive grains;The oil may be used as such or thinned with a solvent-such as vamolene,or'v high flash naphtha, and vcoated at a temperaoi'v the-China-woodoilused. and the amount of drier usedtherein. Where heavy blownv China woodoil lsused and the longer drying time canbe f. allowed, we prefer to usenodrier but where it becomes necessary to decrease the time of drying,about .3 to 1% oi' a manganese or cobalt drier may be addedto-the oil. l

After the treated paper has been dried'. and

., cured it is removed from the racks in the form; ofrolls, and is thenready i'or applicationoi' the.

binder for the grits. Either of. thebinders given byway of illustrationmay be used and a suitable concentrationrior this grit number may bepre-v o,pared by thinning the binder as prepared to a;

solids content of about 67%, with a solvent such as highiiash naphtha.-A suitableA temperature' for application' of the binder is 160` F; andthe l,

binder may be applied with conventional sand'-y paper coating rolls.After'the binderis applied,. alayer of the grit, in this case #320siliconcar` bide. isappliedto the binder in any of the conventional waysbut we prefer to use the new electro-static methods'disclosed in thebefore u mentionedpatents to Elmer C'. Schacht.

After'the grit has been applied, the coated webV is carriedinto aconventional drying room and:

dried ior a. period of abouty 24. hours'at a terni perature 011160? F.toa temperature of` 190 F.

75Atter the binder coat has been cured, a second such asthat representedby a Gardner-Holdt vis-v A- suitable paperbacking is 70 lb. kraft papervturezot. about 140 F. The papertreatment may'v be-applied with.conventional coating rolls audi the paper.` is thenprei'erably passedinto maand-- paper. drying room and dried in testoons at a.temperatureoi' about 130 F. for a period of fromy about41/2 to 24 hours,depending upon the nature or sizing coat of adhesive is applied whichmay be the same binder as used for the making coat or a somewhatdifferent binder. In any case, however, the sizing adhesive should bethinned to a.

lower viscosity than the making coat of binder. A suitable sizingadhesive for grit 320 may be prepared by thinning either of the bindersgiven by way of illustration to a solids content of about 52%, with highflash naphtha. i The sand sizing coat is applied with conventionalsandpaper coating rolls, and a suitable temperature for the applicationof the sand sizing coat is about 140 F. After the sand sizing operation,the coated web is moved into a conventional sandpaper drying room andcured in the form of festoons. Suitable curing times for the item givenby way of illustration are 24 to 36 hours, and suitable curingtemperatures are from 16o-,190 F.

After the sand size has been cured and the making coat has been furthercured after sizing, as indicated, the goods are removed from the racksin the form of rolls. A product so made when cut up into conventionalsheets, belts or discs will show a sand concave curl and this has beenthe customary result before my invention, which contemplates theshrinkage of the cellulosic backing to the approximate dimensions of theadhesive-abrasive coating or to slightly lower dimensions. We maymaterially decrease the sand concave-curl, or produce instead thereof, aflat sand surface or a sand convex-curl by superirnposing upon theprocess Just described, our special treatment. One method of applyingour treatment is to start with goods in the curing racks after the gritshave been applied to the making coat of adhesive, that is to say, duringthe curing of the goods after sand application and before the sandsizing adhesive is added. From experience in the manufacture of thisitem by conventional methods or by a deliberate processing of a smallquantity of goods by conventity of Water vapor in the drying racksaccordtional methods, the approximate extent to which the backing swellsand to which sand concavecurl results therefrom may be determined.Having determined the extent of curling of the paper subsequent tocoating, we add to the drying racks and maintain during the dryingperiod, a quantit-y of a swelling agent, e. g., moisture, such as watervapor in the form of steam or water vapor generated by atomizingor-other suitable means, found necessary to expand, or swell the paper,or to keep the backing expanded to a point equal to or slightly greaterthan that to which it has been found to expand after heating through thecuring cycle and later exposing to average atmospherlc conditions.Usually it will be found satisfactory to measure the width of theuncoated paper when in equilibrium with average atmospheric conditionssuch as 50% R. H. and then maintain in the drying racks during thecuring operation, enough water to prevent shrinkage of the paper, thatis, enough water to maintain the width of the paper equal to or greaterthan its uncoated width when in equilibrium with a relative humidity of50% and a temperature of 70 F.

After the sand size is applied and during the curing operation whichtakes place after sand sizing, water vapor should be added to the dryingracks in sufilcient quantity to prevent shrinkage of the paper. Anindication that this is being accomplished will be theffactthat thecoated paper web is substantially fiat and relatively free from curlingin the racks, especially free from sand convex-curling.

ing to the result that is desired. Greater quantities of water vapor orhigher humidites give a greater swelling of the paper and a greatercorrection or reversal of the sand concave-curl of the finished productto produce a relatively at sheet or a sand convex-curl, whichever ispreferred for the particularvproduct in question.

When the coated abrasive used by way of illustration is removed from theracks and allowed to come to equilibrium, with average atmosphericconditions such as about 70 F. and 40% to 50% relative humidity, therewill be little change in I We have also discovered that at the hightemperatures required to cure certain binders such as thephenol-formaldehyde binders, substantially complete replacement of theair with superheated steam, does not result in a material expansion ofthe paper. In other words, paper and similar cellulosic backings are notmaterially swelled by dry steam (or superheated steam) at temperaturesof around 110 C. or higher.

AA more generally applicable variation of our process of controlling thecurl of coated abrasives made with a cellulosic backing and anon-hygroscopic adhesive consists in shrinking the cellulosic backingafter the coated abrasive has been made in the usual way or according tothe methods disclosed in the patent to Nicholas E. Oglesby, No.2,184,896, issued December 26, 1939.

We may also correct the curl of the coated abrasive used by way ofillustration in the foregoing Example 1 by shrinking the backing afterthe coated abrasive has been fabricated. Where this variation of ourmethod of controlling curl is employed, the coated abrasive is madeor-cured as previously described except that it is not necessary tocontrol the dimensions of the web during curing after grit applicationand after sand size application by the introduction of relatively largequantities' of water vapor into the curing racks. After the final curingoperation the coated abrasive web is taken down in the form of rolls.Just prior to taking down in the form of rolls,l or conveniently duringthe taking down process, or subsequent to the taking down in rolls, weapply a controlled amount of a swelling agent such as water or othercellulose swelling liquid to the back of the abrasively coated web, thatis, to the side opposite to that coated with sand. A convenient methodof applying water to the back of the web is by a set of spray nozzlesspraying into the crotch of the roll at the takedown where `the roll isbeing wound up. We may also use a felt roll which runs in a water troughand touches the back of the coated abrasive web as it is variation ofour invention is shown in Figures 4 and 5. In both figures the samenumbers refer to identical parts.

the direction of the arrows. 2 are idler rolls used to guide the web.The numeral 3 represents friction drums used to regulate the tension onthe web. Numeral 4 represents additional idler rolls used to guide theweb and 5 represents a roll of coated abrasive being wound up in theusual way. 6 represents a manifold connected by a pipe 1 to a waterline, not shown,y and having a multicplicity of spray nozzles 8 whichare directed upwardly against the backing ofthe coated abrasive web.'I'he water is under any suitable line pressure which may vary accordingto the type of spray nozzleused. Many commercial spray nozzles will befound satisfactory. Instead of having the manifold 6 positioned betweenthe rolls l, in some cases, we position it so as .to spray into thecrotch' formed between the web i and the roll 5, as shown in dottedvlines at 9. Generally, we prefer to apply the spray in close proximityto the crotch as this prevents evaporation of the water prior to rollingup and soaking in. If the water is applied insuch positions it will alsousually be found easier to roll up the web.

We may also rst take the roll of coated abrasives down and in asubsequent re-rolling operation, apply Water to the back ofthe coatedweb, or

' wevmay add water' to the back of the coated web by any convenientmeans prior to the taking` down operation, after which 'the web may bedried and rolled up or rolled up and subsequently dried. The watershould be uniformly applied to the backing and allowed to soak in. Afterthe water has soaked into the web; e. g., after about two.

hours, the web may be dried as by rehanging in festoons or anyconvenient method, or the coated abrasive may be cut into sheets, discs,or other forms, and these final forms of the coated abrasive may beallowed to dry.

lWe vary the water added to thebacking according to the product beingmade and according to the extent to which we wish to reverse the normalsand concave curl. Usually water to the extent of 10% to 30% of theweight of the cellulosic backing will be found effective. Within limits,thecorrection of curl will vary with the amount ofv water added to thebacking, but once 25 to. 35% of water, based upon the weight of thecellulosic backing, has been added to the coated web, the use ofadditional quantities of water will usually serve no useful purpose inincreasing the correction or reversal of curl but may be a,disadvantageLas a greater quantity of water will have to be removed inthe subsequent drying operation, thusincreasng the dimculties and theexpense of the drying operation. Furthermore, the use of excessivequantities of water may result in undue softening of the backing; e.-g., paper, making it difcult to handle without tearing or other injury.

Coated abrasive Example 2 i is a coated abrasive web being drawn fromthe curing room, not shown, in

natured) 40 parts by weight to dissolve the condensation product andcool the mass.`

If a lower viscosity is desired, more solvent may be added or converselyless solvent may be used for higher viscosities.

The resin itself when made in accordance with Coated abrasive Example 2if substantially cornpletely dehydrated is a solid atordinarytemperatures. The resin of Coated abrasive Example 2 isremarkably free of penetration into the base at coating concentrationsand, therefore, when this resin is used, there is no objectionablebrittlizing due to impregnation.

The foregoing resinous adhesive may be used in the manufacture of grit0-80 fused aluminum oxide paper as described in the patent to NicholasE. Oglesby, No. 2,184,896 issued December 26,

1939. For this purpose. the resin is prepared to contain approximately75% solids and 25% solvent such as alcohol. y

The adhesive is applied to a backing such as 130 lb. rope cylinder paperat a temperature of about 1z0-130 F. and the grits previously heated toabout 158 F. are applied to the adhesively coated web by electro-staticmeans, all as shown in detail in the said co-pending application. Thecoated abrasive web, while traveling, is then, if necessary, treated asfor example by evaporation to increase the viscosity of or rigidity theresin bond, after which it passes into a coated abrasive drying roomwhere a criticalicuring cycle is applied. A suitable cycle for thisparticular grit The sizing coat may be satisfactorily applied at atemperatureof around rI0-80 F.

After the sand sizing operation, the coated I web is again passed to asandpaper drying room and festooned. A satisfactory drying cycle forthis particular product after the sizing operation is one hour at roomtemperature, about 75 F.. then eighteen hours at 160 F., followed by oneand three-quarters hours at C.

The initial low temperature curing conditions insure the maintenance ofthe grits in substantially their initially applied positions and the nalhigh temperature develops toughness and the tensile strengthof thebinder.

It will be appreciated that many variations can be made in the process,provided the essential and critical conditions are maintained. Theviscosity of the adhesive during and immediately after sand application,as well as the temperature provided for drying and curing the resinsubsequent to the cooling operation which takes place after the sandapplication, are all critically controlled as described in the saidpatent to Nichollas Oglesby, No. 2,184,896, issued December 26, 'I'hegoods made and cured as described may now be rolled up while hot in theform of a roll with conventional takedown equipment. The,y web may nowbe passed between coating rollsv and a thin coating of water may beapplied to the back, that is, the side opposite to that Vto which theabrasive'grain is applied. Instead of v using coating rolls we may usesprays to apply the required quantity of water or othercelluloseswelling liquids, or we may pass the web into an atmosphere ofcondensing steam or otherwise add the required quantity of water. Afterwetting the back the web is again rolled up and the water is allowed topenetrate 'evenly throughoutthe cellulosic backing. The quantity ofwater coated to the backing is governed by the result that it is desiredto obtain and must be varied according to the nature of the cellulosicbacking used, and cold or'hot water may be used. Hot water is normallymore rapid in penetrating and swelling the paper. In the presentvexample where a normal or usual 130 ib. cylinder sandpaper, as used inthe coated abrasive industry, is employed as a backing, water may beadded so that the total water in the cellulosic backing varies fromabout 10% to about 35% according to the result that it is desired toobtain.

After the water or other suitable liquid has been allowed to thoroughlypenetrate into the paper, the web may be rehung and dried orit may becut into sheets, discs, narrow strips or rolls or other forms desiredfor final use and these nal forms may be allowed to dry or be dried.

According to the amount of water added and the treatment of the webafter adding water, there will be a greater or less correction orreversal of curl. Everything else being equal the reversal of curlwillbe greater the greater the quantity of water added up to about atotal moisture content of 30%. The addition of greater quantities ofwater will normally serve little or no useful purpose in changing curlbut will increase the diillculty of handling the web and of the dryingoperation. I

Furthermore,v if the coated abrasive is dried under tension, as forinstance while held nat with weights. or whenrolled up into a. roll sothat it cannot curl on an axis parallel to the machine direction, thereversal of curl will be less than if the coated abrasive is allowed todry free without tension. As typical of one set of results obtained inthe corrections or reversal of curl, -80 alumlnous oxide coated abrasivemade on 130 lb. cylinder paper backing as described in this example wastreated with varying quantities of water and was then dried whilewrapped up in a roll. The width of the web was After the web was dried.short pieces were cut and exposed to an atmosphere of relative humidityat a temperature of about 70 F. and tested for curl characteristics. Inall cases the normal lsand concave curl had been corrected. Where thepaperbacking was treated so as to contain about 10% moisture, the stripscame to equilibrium and produced a slight sand convex-curl, the segmentof a circle formed by a vCoated abrasive Example 3 solvelthe sodiumhydroxide.. Then cool to- 50 C.

and add slowly while stirring, paraformaldehyde, 30 parts by weight.

While stirring raise the temperature slowly over about 70 to 'I5 minutesto about 120 C. and holdat 100-120 C. for about half an hour or untilcooled sample is of desired viscosity. The mass is not allowed to boilso that the reaction will not become violent and uncontrollable.

Instead of using paraformaldehyde as illustrated in Example '1, we mayalso use formaldehyde and secure equivalent results. In still anothervariation, a part or all of the formaldehyde may be replaced withhexamethylene tetramine.

Resin Example B.-A satisfactory method oi.' preparing a resin with alower viscosity than Example A is as follows:

Heat together to 100-110 C. while stirring phenol, 100 parts by weight,and sodium hydroxide `(solid) .86 part by weight, and hold at 100- 110C. for 15 minutes to dissolve the sodium hydroxide. Then cool to 50 C.and add slowly while stirring, paraformaldehyde, 30 parts by weight.

Apply a slight vacuiun (about V2" to 1") and raise the temperatureslowly over one to one and one-quarter hours to 100 C. Raise vacuum to10-15" until temperature is about 70 C. Hold at this temperature undervacuum until cooled gnile shows desired viscosity. Then cool to Theforegoing resin examples consist of Example A, a very viscous resin, andof Example cross section of the 10" strip in the cross direction of thepaper and a line drawn from one edge of the curled paper to the other(dotted une in Figure a) having an altitude (n in Figure 3) of about 1;where the paper was treated to have a moisture content of about 15% thealtitude of the segment was about 2/2": where the paper was treated tohave about 26% water, the altitude of the segment was'about 3".y All ofthese are slight but not objectionable sand con- Vex-curls. At higherhumidities than 25% the sandconvex curi is less. the foregoing mild sandconvex curls at low humiditics being considered typical of a productdesirable for many purposes.

B, a less viscous resin. These two resins may be blended to give avariety of viscosities useful for coating a number oi' coated abrasiveproducts.

These resins may for example be used in coating #24 fused aluminousoxide abrasive. A fabric backing may be selected from various types ofpaper and combinations of cloth and of paper and of cloth andvulcanized" fibre. For the manufacture of #24 fibre backed discs with alresinous binder, a preferred backing isfdescribed in a co-pendingapplication of yOglesby;Reilly and Gilbert, Serial No. 124,506, filedconcurrently herewith. .y

The preferred backing consists of one lamina oi vulcanized fibre ofabout 10 mills in thickness and weighing about 200 lbs. per paper reamto -action known as vulcanizing which consists of an appropriatetreatment with sulphuric .acid or more commonly with a solution of zincchloride, whereby the cellulose is more or lessv gelatinized butgenerally without complete disintegration o! the individual bres. Afterthe gelatlnizlng action, the web is washed thoroughly to remove alltraces of sulphuric acid or zinc chloride as the case may be, as thepresence of even minute quantities of these materials will result in arapid I deterioration of the product as, for example, loss of strength.

The combination web prepared as described and as given in further detailin the said co-pending appli-cation is removed from the curing racks andis ready for coating. While either side of the resin may be coated, wegenerally prefer to coat the cloth side of the combination.

A suitable adhesive for the manufacture of #24 aluminous oxide isillustrated by Resin Example A or by a mixture of Resin Example A andResin Example B. A diagrammatic illustration of equipment suitable forprocessing this item with these resins is shown in the patent toNicholas E. Oglesby, No. 2,184,896, issued December 26, 1939.

The adhesive used for anchoring the grits to the backing (the makingcoat) may be Resin Example A or Resin Example A thinned with a CII smallquantity of Resin Example B. The adhesive is applied with coating rolls,as for instance, at a temperature of G-160 F. A suitable weight ofadhesive for this particular case is about 22 lbs. per sandpaper ream.The adhesively coated web passes under a sandfall where an excess ofgrits is applied at a tempera-ture of about 150 F. After the sand isapplied in excess of that required for the coating, the adhesivelycoated web carrying some excess sand passes into nip rolls where thesand is pushed into the adhesive. The coa-ted web may then pass around asand drum where there is a further pushing in of the abrasive grains.The coated Web then passes over a suction drum which is used to pull theweb through the machine and from the suction drum the web is conveyed byconventional means to a sandpaper drying room where it is festooned andheat is applied through a critically controlled curing cycle.

For the adhesive and method used by* way of illustration, a satisfactorydrying cycle is found to be 105 F. for 2 hours; 110 F. for 1% hours;115u F. for 2 hours; 120 F. for 2 hours; 125 F. for 2 hours and 130 F.for l hours. The goods may then be cooled t-o a temperature of around 70to 80 F. to renderv the adhesive less tacky and then taken down in rollform or.v other conventional ways or if desired the web may be moveddirectly to the sizing apparatus and sized. A preferred method is totake down in the form of rolls and mount the rolls on a suitable bundlestand or other means in front of a coated abrasive sand sizing machine,Where the sizing adhesive is applied by coating rolls. A satisfactorytemperature for application of the sizing adhesive used in this case isP10-150 F.

A satisfactory adhesive for the sizing operation may be prepared bytaking 60 parts by weight of Resin Example A and mixing with 40 partsby` weight of the less viscous resin illustrated in Example B. For manyapplications, a satisfactory weight of the sizing material to be appliedis 27 lbs. per sandpaper rearn. After the sizing coat has been applied,as described on conventional sandpaper equipment, the sand sized web isthen delivered by the usual means to a sandpaper drying room Where it isfestooned and where a heat cycle is applied. This cycle is critical topreserve orientation, the sized goods, as

l explained, being subject to now with consequent In the presentillustration, a satisfactory drying cycle Within the drying room is asfollows: lF.--slx and one-quarter hours; F.ve hours; 'F.-five andone-quarter hours 1'15" F.iour and one-half hours; 120 F.five hours;F.-ve and one-quarter hours and 130 F.- ten and one-half hours. Afterthis drying cycle has been completed the goods should be cooled to atemperature of around 95 F. to reduce the tackiness of the adhesive sothat the goods can be taken down in the form of rolls. Excessive coolingshould be avoided as otherwise the goods will become too brittle forsatisfactory taking down and winding up in rolls.

The goods, taken from the sandpaper drying room in the form of rolls,are then cut into satisfactory lengths or strips for a subsequent dryingoperation to finally cure the resin binder and develop high tensilestrength of about 8000 to 14,000 lbs. per square inch or more. It hasbeen found that a drying cycle of any commercially economical durationat relatively low temperatures will not produce the iinal cure of thisresin which is required for maximum cutting emciency and that the cut ofthe coated material is greatly increased by a high temperature dryingcycle now to be described.

The strips cut from the rolls of coated goods are conveniently cured tothe final condition by placing them on suitably arranged shelves withina baking oven which can be regulated within critical limits to obtainthe desired temperatures and temperature control. Asatisfactory naldrying cycle for the strips is as follows: eight hours- 70 C.; eighthours-80 C.; then gradually raise the temperature to 100 C. so that atemperature of 100 C. is reached within one hour; hold at 100 C. forvone hour; raise during the next hour to a temperature of 120 C. andhold at 120 C. for one hour; during the next hour raise gradually to C.and then hold at 140 C. for one hour; during the next hour raisegradually to C. and then hold for three and one-half hours. Turn oil theheat. open the oven and allow the strips to cool. The strips are thenremoved and are subsequently cut into conventional disc shapes or anyother iorm that it is desired to produce as an abrasive article.

The strips as removed from the iinal high temperature bake are dry andrelatively brittle Prior to cutting into the form of discs they shouldbe stored under light pressure to prevent excessive curling until theyhave taken up enough moisture from the atmosphere to reduce thebrittleness of the dry backing, or a small quantity of water should beapplied to the backing if it is desired to cut the strips before theyhave had time to pick up sufficient moisture from the atmosphere. Inthis case it is only necessary to apply water to the extent of about '7or 8% of the weight of the vulcanized bre in the backing, since in theprocess of combining and coating the cloth has become substantiallyimpregnated with the resinous adhesive and is no longer subject to rapidchanges in moisture content when exposed to variable humidities. Waterto the extent of about 7 or 8% of the Weight of the vulcanized fibre maybe applied to the backing of the strips in any convenient way as bysponging, painting, spraying or passing through soft coating rolls asfor instance, felt rolls, one of which turns in av trough of water.After the application of this Water the strips should be stacked for aperiod of time to allow the water to soak in'to the vulcanized nbre;usually a period of time of two hours or more will suflice.

The strips are then cut into discs in the conventional way, and thebacks of the discs are treated with a further quantity of water to bringthe moisture content of the vulcanized fibre to from l-25% of itsweight. 'Ihls water may be applied by means of a spray, by sponging, bypainting, or by coating rolls or by other suitable means. The quantityof water added to the backs of the discs is regulated in accordance withthe result that it is desired to obtain. The greater the quantity ofwater applied, the greater will be the correction or reversal of thesand concave curl, to produce a disc which is relatively flat at averagetemperatures and humidities or a v disc which under these conditionswill show a desired and limited degree of said convex-curl.

Immediately after the backs of the discs have been wetted, the discsshould be stacked one on top of the other and baled, as for instance, byapplying end boards to each end ofva stack of discs, placing in a balingpress, pressing and fastening wire around the end boards. The pressureapplied in the baling operation should not be enough to damage the discsto any material extent due to pushing the grains into the vulcanizedfibre where the abrasively coated side of one disc comes in contact withthe vulcanized fibre side of the other disc. The pressure should,however, be great enough to keep the discs substantially at. The balesof discs preferably should not be wrapped or made impervious tocirculation of air until after storage at ordinary atmosphericconditions for about one week. During this time a substantial part ofthe excess water in the backs of the discs will evaporate. Allowing thediscs to dry while held under light pressure prevents warpage duringdrying and in fact materially reduces any warpage that may have occurredpreviously during the processing of the coated abrasive. Furthermore, ifthe discs are dried under Lght pressure and if by error excess water hasbeen applied to the backing, the chance of excessive sand convex-curl inthe final coated abrasive discs as used willbe minimized. It should beappreciated that where the cellulosic backing such as paper orvulcanized fibre has been treated with a given substantial quantity ofwater, the reversal of curl will be less if the water-treated coatedabrasive is allowed to dry at under pressure than will be the case ifthe same or similar coated abrasive is allowed to dry without restraintand is therefore able to curl at will during the drying.

After the water-treated discs have been allowed to stand at normalatmospheric conditions for about one week, they may be wrapped in anyway desired for shipment or storage. Where the discs are to be used fora dry sanding operation, no further treatment is required.

If, however, it is desired to use discs of the typo described in anabrading operation where water is used as a lubricant or where the workis otherwise wet or under conditions whereby the disc might become wet,the backs of the discs should be given a waterproofing treatment torender them suitable for wet operations. We have found that a coating ofparaflin emulsion will render the discs suitable for most wet abradingoperations, but wemay also waterproof the backing with various flexiblevarnishes, lacquers, e. g.. nitrocellulose or cellulose acetate withsuitable plasticizers, or resins, e. g., Rezyl No. 408 above mentioned,all of which are commercially available. For some purposes, a coating oftung oil or linseed oil will be effective.

Where it is desired to waterproof the discs with a paraffin emulsion, wemay add a paraiiln emulsion in water to the backing of the discs in lieuof the pure water treatment used to correct the curl of the discs,thereby combining the control-of-curl step with the waterproofing step.

In the examples given by way of illustration, details have been givenfor the control or correction of curl by the use of a swelling agent forcellulose, e. g. water either in the form of vapor or as liquid. We mayuse other agents which are capable of shrinking cellulosic backings ftothe dimensions of the adhesive-abrasive coat or to dimensions which areslightly smaller than the abrasive-adhesive coating to produce arelatively fiat coated abrasive or a coated abrasive with a controlleddegree of sand convex-curl instead of the normal sand concave-curlobtained with coated abrasives made with a nonhygroscopic binder.

The rate at which a liquid penetrates into a cellulosic backing isgoverned to some extent by the surface tension of the liquid. Theorganic compound methyl alcohol readily penetrates cellulosic backingsand has been found to function in a manner analogous to the behavior ofwater. Furthermore, methyl alcohol is quite volatile and is more readilyremoved from the backing than water. A suitable grade of methyl alcoholis methyl alcohol, the remainder being mainly water.

Mixed liquids such as a mixture of ethyl alcohol and water or a mixtureof acetone and water may also be used to control or reverse the curl ofcoated abrasives made with a water resistant or non-hygroscopic binderby applying these liquids to the backings of the previously fabricatedcoated abrasives. A suitable mixture of ethyl alcohol and water may beprepared by taking one volume of ethyl alcohol and one volume of water.Similarly, a mixture consisting of one volume of acetone and one volumeof water may be used. These mixed solutions of alcohol and water or ofacetone and water have been found in some instances to penetrate intoand swell cellulosic backings more rapidly than pure water. Furthermore,volatile solvents such as acetone and alcohol are more quickly andreadily evaporated from the backing than pure water.

The use of mixed liquids such as alcohol and water or of acetone andwater in some cases makes it easier to control the extent of thereversal of curl in that a larger volume of the liquid as compared withpure water, may be applied to the cellulosic backing without acorrespending increase in the sand convex-curl obtained in thesubsequently dried product.

It is believed that liquids which are suitable for the control of thecurling of coated abrasives function by softening and swelling thecellulosic fibres and loosening the natural cementing cellulosicadhesive to an extent that some realignment of the bres to relievestresses and strains is possible. Regardless of the mechanism by whichthe correction takes place, the net result is a shrinkage of thecellulosic backing whereby sand concave curl is corrected and there isproduced instead thereof. a relatively flat coated abrasive or a coatedabrasive with a controlled and desired degree of sand convex curl aboutan axis parallel to the machine direction in the case of paper,vulcanized fibre backings or similar water-laid sheet.

As explained before, open coated abrasives made with a glutinous orsimilar hygroscopic binder and conventional paper, curl sand concave.Ordinarily it is not advisable to correct this curl by wetting thebacking with water since wetting and subsequent drying results in somedamage to the bond between the glutinous adhesive and the abrasivegrains. Where correction of curl of open coat abrasives made with aglutinous binder is required, we may treat the backing with methylalcohol, thereby doing less damage to the bond between the glutinousbinder and the abrasive grains than is obtained where water is used fora similar purpose.

While we have referred herein to binders of the synthetic resin type andthese are preferred. it is to be understood in some cases,4we may use avarnish prepared from China-wood oil. rosin, suitable thinners, anddryers as is well known in the varnish making art. In like manner, -insome cases, we may use a solution of shellac and alcohol andsubsequently heat the shellac to harden and toughen it, and we may alsoadd to the shellac suitable plasticizers.

Natural resins have heretofore been used as components of binders in thecoated abrasive art. The term binder as used in this application isintended to cover the various adhesives mentioned which arenon-hygroscopic, as well as other binders which exhibit thischaracteristic.

In accordance with the present invention, there is obtained a coatedabrasive in which the grit is adhered by a synthetic resinous bond andthe backing may consist of paper, vulcanized fibre or cloth or beconstituted by a combination of these materials united by a suitablebinder preferably one of the synthetic resin type. It is an importantcharacteristic of the preferred adhesive for the grit that the tensilestrength thereof is above 6,000 lbs. per square inch and usually between8,000 and 14,000 lbs. about persquare inch. Further, the preferredsynthetic resin bond for the grits is resistant to temperatures up tosubstantially 500 F. and is not affected by water or organic solvents.It is another important characteristic of the invention that the producthas a substantial volume of the individual grits exposed above theaverage level of the adhesive layer which is free to engage the work,usually about 1/3 to 1/2 or more of the total volurne of the grits willbe above the average level of the adhesive layer. Also, it is preferredthat between the base of the grit layer and the adjacent surface of thebacking there is a deflnite continuous layer of the synthetic resinadhesive. These characteristics impart to products Amade in accordancewith this invention a very high efficiency.

We claim:

l. In the manufacture of coated' abrasives having a cellulosic backing,a synthetic resin adhesive coating thereon, and a layer of abrasive gritheld by said adhesive, the step of treating the backing with a swellingagent therefor sufficiently to produce nal backing dimensions of suchrelation to the dimensions of the abrasive adhesive coating that thecoated abrasive has a grit surexposing the backing to an agent effectivein swelling the same to an extent that the said dimensions in the nalproduct along the surface of the backing are equal to or slightly lessthan the dimensions of the abrasive adhesive coating at average relativehumidities and temperatures in common conditions of use, and the coatedabrasive will either be relatively flat or show a controlled and desiredamount of sand convex-curl 3. In the manufacture of coated abrasiveshaving a cellulosic backing, a synthetic resin adhesive coating thereonand a layer of abrasive grit held by said adhesive, the step of curingthe binder in the presence of an atmosphere containing moisture andsubjecting the backing to the moisture sufficiently to produce finaldimensions thereof in relation to the adhesive-abrasive coating as toproduce a final grit surface character which is substantially fiat toconvexcurl and does not become concave under common variations inrelative humidities and temperatures.

4. In the manufacture of coated abrasives having a cellulosic backing, asynthetic resin adhesive coated thereon, and a layer of abrasive gritheld by said adhesive, the step, after the coating has been applied andat least partially cured, of shrinking the backing sufficiently toproduce a backing of such dimensions in relation to theadhesive-abrasive coating dimensions that it will not under averageconditions of use absorb sufficient moisture from the atmosphere to curlthe article and present a concave abrasive surface, thereby formingl acoated abrasive having a grit surface character substantially fiat'toconvexcurl under average conditions of use. whereby a concave gritsurface is avoided.

5. In the manufacture of coateld abrasives having a cellulosic backing,a synthetic resinous adhesive coating thereon, and a layer of abrasivegrit held by said adhesive. the step of curing the binder in thepresence of a swelling agent for the backing sufficient to maintain thewidth dimension of the backing during curing of the resinous binder atleast equal to the width dimension of the backing before coating underaverage conditions and to produce a backing of such final dimensions inrelation to the adhesiveabrasive coating that under average conditionsof use it will not absorb sufficient moisture from the atmosphere tocurl the article to form a concave abrasive surface, whereby a concavegrit surface is avoided.

6. In the manufacture of coated abrasives having a cellulosic backing, asynthetic resin adhesive coating thereon, and a layer of abrasive gritheld by said adhesive, the step, after the coating has been applied andhardened, of shrinking the backing sufnciently to produce a backing ofsuch dimensions in relation to the dimensions of the adhesive-abrasivecoating that under average conditions of use it will not absorbsufficient moisture to curl the abrasive to present a concave abrasivesurface, thereby produclng av coated abrasive having a grit surfacecharacter substantially fiat to convex-curl under average conditions ofuse, said shrinking being effected by subjecting the backing to anaqueous agent effective to shrink the same but not materially injuringthe bond between the grit and the adhesive.

7. In the manufacture of coated abrasives having a cellulosic backing, asynthetic resin adhesive coating thereon, and a layer of abrasive gritheld by said adhesive, the step, after the coating has been applied andhardened, of shrinking the backing sufficiently to produce a backing ofsuch dimensions in relation to the dimensions of the adhesive-abrasivecoating that under average conditions of use it will not abcorbsufficient moisture to curl the abrasive to present a concave abrasivesurface, thereby producing a coated abrasive having a grit surfacecharacter substantially fiat to convex-curl under average conditions ofuse, said shrinking being effected by subjecting the backing to anorganic substance effective to shrink the same but not materiallyinjuring the bond between the grit and the adhesive.

8. In the manufacture of coated abrasives having a cellulosic backing, asynthetic resin adhesive coating thereon and a layer of abrasive gritheld by said adhesive, the steps of shrinking the backing suiciently toproduce a backing of such dimensions in relation to the dimensions ofthe adhesive-abrasive coating that under average conditions of use itwill not absorb sufcient moisture to curl the abrasive to present aconcave abrasive surface, thereby producing a coated abrasive having agrit surface character substantially nat to convex-curl under averageconditions of use and applying a waterproof coating to said backing.

9. In the manufacture of coated abrasives having a cellulosic backing, acellulosic backing, a synthetic resin adhesive coating thereon, and alayer of abrasive grit held by said adhesive, the step, after thecoating has been applied and at least partially cured, of shrinking thebacking suiiciently to produce a backing of such dimensions in relationto the dimensions of the adhesive-abrasive coating that under averageconditions of use it will not absorb suiTlcient moisture to curl theabrasive to present a concave abrasive surface, thereby producing acoated abrasive having a grit surface character substantially nat toconvex-curl under average conditions of use, said shrinking includingthe wetting of the backlng with a volatile agent which serves to swellthe cellulose and subsequently drying the backing.

10. In the manufacture of coated abrasives having a cellulosic backing,a synthetic resin adhesive coating thereon and a layer of abrasive gritheld by said adhesive, the step of shrinking the backing suiiciently toproduce a backing of such dimensions in relation to the dimensions ofthe adhesive-abrasive coating that under average conditions o f use itwill not absorb suflicient moisture to curl the abrasive to present aconcave abrasive surface, thereby producing a coated abrasive' having agrit surface character substantially fiat to convex-curl under averageconditions of use, and waterproofing said backing, said step involvingthe use of an agent eective both to shrink the backing and to waterproofthe saine.

11. A relatively stiff but flexible abrasive disc asoavii for use inconnection with a yieldable supporting pad against which the disc isheld by attachment at the center only, said disc comprising a stiff,resilient backing having a thin layer of abrasive grains attached to asurface thereof by a heathardened resinous binder, and being circularwhen pressed to form a plane surface but normally curved so that aparticular diameter will be a substantially straight line while adiameter intersecting the first-mentioned diameter at an angle ofdegrees is substantially an arc of a circle, whereby the disc issufficiently compressed when mounted for use to cause it to lie snuglyagainst the supporting pad.

12. A relatively stiff but flexible abrasive disc for use in connectionwith a yieldable supporting pad against which the disc is held byattachment at the center only, said disc comprising a stiff, resilientbacking having a thin layer of abrasive grains attached to a surfacethereof by a heat hardened resinous binder, said abrasive binder coatingbeing of substantially constant area throughout a range of substantially15% to 60% relative humidity at substantially '70 F., said backingincluding a cellulosic material and being of a variable area throughoutsaid humidity range but being so dimensioned in area in relation to theabrasive binder coating that it is normally of less area than theabrasive binder coating throughout said humidity range whereby the discalthough circular when pressed to form a plane surface is normallysubstantially throughout said humidity range convexly curled on itsabrasive binder surface so that a particular diameter is a substantiallystraight line While a diameter intersecting the first mentioned diameterat an angle of 90 is substantially an arc of a circle, whereby the discis suiclently compressed when mounted for use as to cause it to liesnugly against the supporting pad.

13. An abrasive disc comprising a stiff. resilient backing having a thinlayer of abrasive grains attached to a surface thereof by a heathardened phenol-aldehyde resinous binder, said abrasive binder coatingbeing of substantially constant area throughout a range of substantially15% to 60% relative humidity at substantially 70 F., said backingincluding a cellulosic material the area of which is subject to changein response to variations in humidity but normally of less area than theabrasive binder coating throughout said humidity range whereby the discis circular when pressed to form a plane surface and in the absence ofsuch pressure is normally, substantially throughout said humidity range,convexly curled on its abrasive binder surface so that a particulardiameter is a substantially straight line while a diameter intersectingthe first-mentioned diameter at an angle of 90 is substantially an arcof a circle, the disc being sufficiently compressed when mounted for useas to cause it to lie snugly against its supporting pad.

NICHQLAB E. OGLESBY. FRANKLIN S'IRAIN.

